ETA relevant links: https://youtube.com/@NotJustBikes https://letgrow.org/
ETA again: I glibly mentioned "being killed by a driver" but of course navigating the typical US built environment if you're under 16 or otherwise unable to drive is a miserable experience in a number of ways even if you survive it. Highways make pedestrian paths unnecessarily roundabout. Parking lots make everything further from everything else. Crossing major roads requires getting drivers to notice and stop for you (harder when you're short!), or waiting through interminable signal cycles, etc.
It may depends on where in the US you're talking about, but in my area none of this is actually true. Although lots of people believe it is.
I live in a suburb in California and have never ran into this problem. But we're also unincorporated and don't have cops with nothing better to do than harass some kid walking to school alone.
We're also surrounded by people whose kids are now in their 20s-30s. They don't see our kids running around as a nuisance - they're relatively new empty nesters and the kids seem to evoke nostalgia as they discuss the different things their kids did around the neighborhood when they were young.